The present invention generally relates to an improved electrographic plotter and method for improving the performance of electrographic plotters. More specifically, the present invention relates to a method and apparatus to prevent fountain image ghosting on FIGURE plots written on film medium.
Electrographic plotters are known in the prior art, and FIG. 6 is a schematic illustration of the operation of an electrographic plotter 10. Plotter 10 typically has a spiral wound roll of a recording medium 12. Recording medium 12 is taken off the roll and passed in the direction of arrow 14 between a recording head 16 and a plurality of backplate electrodes 18. Recording head 16 includes a plurality of nibs 20 which, in conjunction with backplate electrodes 18, comprise a means for depositing charged particles onto recording medium 12 in a desired pattern. In the particular embodiment described, negatively charged particles are deposited. The pattern of deposited charges is referred to as a latent image. Recording medium 12 is next taken past a developing station 30, also called a fountain, which contains charged particles of a polarity opposite to the deposited particles.
In operation, an image I to be plotted is coded as a digital signal (sequence of 1's and 0's) and sent to a driver circuit 32. Driving circuit 32 operates to control recording head 16 and backplate electrodes 18 to establish an electric potential across recording medium 12. The potential is established by pulsing selected nibs 20 of recording head 16 in a desired pattern as medium 12 passes along the recording station. The typical voltage level between nibs 20 and backplate electrodes 18 for recording an image is 500-600 volts. This causes an electrical discharge. The image itself deposited by the nib-to-media discharge is 100-200 volts. Thus, as selected nibs 20 are pulsed, a latent electrostatic image of image I is deposited on the underside of medium 12 as medium 12 moves toward fountain 30. Recording medium 12 generally comprises two coatings on a substrate, a dielectric material 100 and a conductive material 102. Dielectric material 100 is oriented on the underside for this particular plotter 10. The latent image is subsequently developed at fountain 30 by any of numerous well known processes wherein pigment particles are positively charged and are brought into contact with the negatively charged latent image and processed. Thus, the latent image is developed and made visible.
To facilitate an understanding of the operation of plotter 10, a brief discussion of the mechanism of pulsing nibs 20 will be made. A latent image is composed of a series of "pixels" (picture elements) arranged across the width of a scan, generally 100-400 pixels per inch. The pixels are deposited in well known fashion by pulsing particular ones of nibs 20. To ensure that solid filling and continuous lines are possible, the areas where successive pixels would write must overlap. To provide for this overlap, nibs 20 are usually constructed in two rows. If nibs were numbered sequentially, there would be a row of the odd-numbered nibs and a row of the even-numbered nibs displaced from that. Images from these rows are recorded in an interleaved manner, as is well known.
FIG. 8 is a perspective representation of nibs 20 arranged in recording head 16. Logically, the nibs are arranged to be serially pulsed, with odd pixels recorded by a first row and even pixels recorded after a short delay when the second row of nibs is over the line of pixels written by the first (odd) row.
Nibs 20 along the head are in groups wired in parallel. By use of well-known multiplexing techniques, one successive group of nibs after another along the head is written until a full line, or "scan", is written by each of the odd and even rows.
Recording medium 12 is a special electrographic paper, film, or other material. For electrographic paper, a base paper 102 is impregnated with a solution to make it conductive. This is then coated with a dielectric layer 100.
Also used in plotters is an electrographic film medium. FIG. 2 illustrates a cross-sectional view of recording medium 12' which is a film medium. Film media is comprised of three layers. A film layer 104, a conductive layer 102, and a dielectric layer 100.
FIG. 4 illustrates fountain image ghosting occurring when film medium 12' is used in plotter 10 to receive the latent electrostatic image. Fountain ghosting is the consequence of fountain 30 located "downstream" of recording head 16. As a prior recorded first image I.sub.1 is being developed at fountain 30, a charge associated with a second image I.sub.2 is deposited on film medium 12' by recording head 16. The writing of the latent image I.sub.2 establishes an electric potential which extends to fountain 30 to cause a ghost image, G.sub.1, to be superimposed or developed at the same time as the first image I.sub.1. Generally, ghost image G.sub.1 is a lighter image than second image I.sub.2 with details less defined. The ghost image G.sub.1 is lighter closer to the plot edges. However, the area of overlap of I.sub.1 and G.sub.1 is darker because of an increased potential over fountain 30.
FIG. 5 illustrates an experimental plot depicting equipotential lines established during a simulated recording of data on film medium 12'. The plot was made by establishing a 100 volt potential at a conductive simulation stripe which is electrically connected to conductive layer 102 placed across the film medium 12' (illustrated in FIG. 2). The position of the simulation stripe corresponds to a recording head position. In typical plotters, fountain 30 may be 4-12 inches downstream of recording head 16. FIG. 5 reveals that equipotential lines of significant voltage extend downstream a significant distance. The reader will understand that the pattern of the equipotential lines will not change with varying voltages applied to the conductive simulation stripe, while the magnitudes of particular equipotential lines will vary directly with the applied voltage. This plot is representative of equipotential lines established by charges deposited during recording.
The cause of fountain ghosting is apparent and may be described by reference to FIGS. 2, 4 and 5 as follows. As a particular latent image is deposited by application of negatively charged particles 110 upon dielectric layer 100, corresponding positive charged particles 112 from conductive layer 102 are associated with them, establishing a dipole. As the negative charges 110 on dielectric layer 100 are relatively immobile, and positive charges 112 are "neutralized" thereby, there is an excess of negative charges 114 in conductive layer 102 which are "un-neutralized." It is desired that conductive layer 102 be grounded, therefore edge stripes 40 are provided for film medium 12'. Edge stripes 40 are formed during manufacture by painting a conductive ink which also dissolves a portion of dielectric layer 100 to make contact to the conductive layer 102. Excess un-neutralized negative charges establish an electric potential which drives the un-neutralized charges out to the grounded edge stripes. The established negative potential extends over fountain 30 and therefore the conductive layer attracts positive toner particles in fountain 30 to a underside of film medium 12'. Thus, a ghost image of a currently recorded image is superimposed upon any latent image currently being developed, or the ghost image is developed over an area desired to be undeveloped.
The ghost image is influenced by many factors. As described above, the ghosting potential is directly related to the rate of charging of the dielectric. Thus, the ghosting potential is directly dependent upon plotter speed, data flow, dark-pot setting (writing voltage), and a dielectric constant of the film medium 12'. The ghosting potential is produced as the un-neutralized charges leak along conductive layer 102 to edge-stripe 40 at ground point 42. Therefore, the ghosting potential is also proportional to the sheet resistance (which usually is dependent upon humidity and temperature).
This ghosting potential over the fountain caused by the potential established to drive charge (released by the writing process) to the edge stripes can be simulated or calculated.
An additional concern with ghosting is the phenomenon referred to by ourselves as anti-ghosting. An anti-ghost is the area upstream of a ghost image. As is appreciated, when a ghost image is superimposed upon a recorded image, the area of overlap will appear to be darker, as a larger potential was used to attract positive charges from the fountain. If the ghost image stops (because the writing stops) as the recorded image continues to be developed, an apparent lightening of the image will result because an unmodified "normal" potential is then solely responsible for the intensity of the image. It is the lighter image, without ghost, which is referred to as an anti-ghost.
It is important to note that because the ghosting potential is caused by the potential drop across the media as released charge passes to the edge stripes, that potential is dependent on the plotter geometry and is proportional to the media resistivity and the rate of unneutralized charge release. The charge release rate in turn is proportional to the written image voltage and the "data flow," that is, the rate of 1's being written.
We have sensed the ghosting potential over the fountain with, such as for example, a kelvin probe, and servo-ed the bias voltage to eliminate this ghosting potential at the center of the fountain.
However, we have found it more convenient and less susceptible to extraneous signals to take into account the plotter geometry and media resistivity by a manual potentiometer setting and to vary the bias voltage according to the dark-pot setting of the plotter (writing voltage) and the rate of writing 1's. The rate of writing 1's, of course, depends on the speed of the plotter and percentage of 1's written in comparison to total pixels written.
The correct bias is not exactly proportional to the rate of writing 1's, because of pixel overlap. When one pixel is written adjacent to another and "overlaps" that pixel, the overlap region is not written twice. The second pixel merely writes in that region not already written to by the first pixel. Thus, separated pixels will release more free charge than clumped pixels, the ghosting potential will be greater, and therefore the required bias will be greater. Thus, there is a rough non-linearity between the required bias voltage and the rate of writing 1's. This non-linear dependence is shown in FIG. 7.
The reader will appreciate that in use of the standard type of paper medium 12 described previously, fountain ghosting will not occur. The reason for this is described as follows. In FIG. 6, the operation of a plotter 10 recording on paper medium 12 is illustrated. While the latent image is being deposited on the underside of paper 12, there are backplate electrodes 18 contacting the top surface of the paper 102. These backplate electrodes 18 are connected to ground through a plurality of backplate drivers (not shown). Thus, after a nib group writes, the corresponding backplates return to ground and any dis-associated negative charges are immediately swept out of the conductive layer. The sweeping out is efficient as access to conductive layer 102 is possible across the entire width of the paper 12. With film medium 12', capacitive coupling is used to record a charge, and direct access to the conductive layer is only possible along edge stripes 40. Therefore, un-neutralized negative particles must leak to the side edge at selected ground points 42.
Although ghosting is therefore not a problem with writing on paper, there is some prior art in writing on paper.
U.S. Pat. No. 4,290,076 entitled "Compensatory Means Improving The Operation of Electrostatic Printers," issued Sept. 15, 1981 to McFarland illustrates a prior art method used to eliminate a potential difference between a conductive layer of paper recording medium and a fountain. A circuit is provided for sensing the potential immediately upstream from the developing station, and to supply a current to the conductive layer to bring the potential difference to zero and therefore reduce a developer "plating-out" problem. The non-zero potential over the fountain is created by recording with backplate voltages above ground, by triboelectric ("rubbing") charging, and by insufficient grounding of the conductive layer between the backplates and the fountains.